Which extra-ocular muscle is supplied by opposite side nucleus?
**Core Concept**
The extra-ocular muscles are controlled by three cranial nerves: **oculomotor (III)**, **trochlear (IV)**, and **abducens (VI)**. Each muscle has a specific innervation pattern. The **trochlear nerve** is unique as it is the only cranial nerve that arises from the dorsal aspect of the brainstem.
**Why the Correct Answer is Right**
The **superior oblique muscle** is innervated by the **trochlear nerve (IV)**. This nerve arises from the **dorsal midbrain** and crosses over at the level of the brainstem, meaning that each **trochlear nucleus** supplies the **contralateral superior oblique muscle**. This crossing over is what allows the muscle to be supplied by the opposite side nucleus.
**Why Each Wrong Option is Incorrect**
**Option A:** The lateral rectus muscle is supplied by the abducens nerve (VI), which does not cross over at the level of the brainstem.
**Option B:** The medial rectus muscle is supplied by the oculomotor nerve (III), which also does not cross over.
**Option D:** The inferior oblique muscle is supplied by the oculomotor nerve (III) and does not receive innervation from the opposite side nucleus.
**Clinical Pearl / High-Yield Fact**
Remember that the **trochlear nerve** is the thinnest and longest cranial nerve, and its unique crossing pattern is a key point for understanding extra-ocular muscle innervation.
**Correct Answer:** D. Superior oblique muscle